Cellulose production from lignocellulosic biomass

ABSTRACT

A multi-function process is described for the separation of cellulose fibers from the other constituents of lignocellulosic biomass such as found in trees, grasses, agricultural waste, and waste paper with application in the preparation of feedstocks for use in the manufacture of paper, plastics, ethanol, and other chemicals. This process minimizes waste disposal problems since it uses only steam, water, and oxygen at elevated temperature in the range of 180° C. to 240° C. for 1 to 10 minutes plus a small amount of chemical reagents to maintain pH in the range 8 to 13. An energy recuperation function is important to the economic viability of the process.

1. RELATED APPLICATION

[0001] The present application is a continuation-in-part of co-pendingU.S. patent application Ser. No. 09/640,815 filed Aug. 16, 2000,entitled CELLULOSE PRODUCTION FROM LIGNOCELLULOSIC BIOMASS, which isassigned to the assignee of the present case.

2. FIELD OF THE INVENTION

[0002] This invention relates to the production of cellulose fromlignocellulosic biomass, and in particular to process whereby celluloseis separated from other constituents of lignocellulosic biomass so as tomake the cellulose available as a chemical feedstock and/or accessibleto enzymatic hydrolysis for conversion to sugar.

3. BACKGROUND OF THE INVENTION

[0003] The possibility of producing sugar and other products fromcellulose has received much attention. This attention is due to theavailability of large amounts of cellulosic feedstock, the need tominimize burning or landfilling of waste cellulosic materials, and theusefulness of sugar and cellulose as raw materials substituting foroil-based products.

[0004] Natural cellulosic feedstocks typically are referred to as“biomass”. Many types of biomass, including wood, paper, agriculturalresidues, herbaceous crops, and municipal and industrial solid wastes,have been considered as feedstocks. These biomass materials primarilyconsist of cellulose, hemicellulose, and lignin bound together in acomplex gel structure along with small quantities of extractives,pectins, proteins, and ash. Due to the complex chemical structure of thebiomass material, microorganisms and enzymes cannot effectively attackthe cellulose without prior treatment because the cellulose is highlyinaccessible to enzymes or bacteria. This inaccessibility is illustratedby the inability of cattle to digest wood with its high lignin contenteven though they can digest cellulose from such material as grass.Successful commercial use of biomass as a chemical feedstock depends onthe separation of cellulose from other constituents.

[0005] The separation of cellulose from other biomass constituentsremains problematic, in part because the chemical structure oflignocellulosic biomass is not yet well understood. See, e.g., ACSSymposium Series 397, “Lignin, Properties and Materials”, edited by W.G. Glasser and S Sarkanen, published by the American Chemical Society,1989, which includes the statement that “[l]ignin in the true middlelamella of wood is a random three-dimensional network polymer comprisedof phenylpropane monomers linked together in different ways. Lignin inthe secondary wall is a nonrandom two-dimensional network polymer. Thechemical structure of the monomers and linkages which constitute thesenetworks differ in different morphological regions (middle lamella vs.secondary wall), different types of cell (vessels vs. fibers), anddifferent types of wood (softwoods vs. hardwoods). When wood isdelignified, the properties of the macromolecules made soluble reflectthe properties of the network from which they are derived.”

[0006] The separation of cellulose from other biomass constituents isfurther complicated by the fact that lignin is intertwined and linked invarious ways with cellulose and hemicellulose. In this complex system,it is not surprising that the “severity index” commonly used in datacorrelation and briefly described below, can be misleading. This indexhas a theoretical basis for chemical reactions (such as hydrolysis)involving covalent linkages. In lignocellulose, however, there arebelieved to be four different mechanisms of non-covalent molecularassociation contributing to the structure: hydrogen bonding,stereoregular association, lyophobic bonding, and charge transferbonding. Bonding occurs both within and between components. Astemperature is increased, bonds of different types and at differentlocations in the polymeric structure will progressively “melt”, therebydisrupting the structure and mobilizing the monomers andmacro-molecules.

[0007] Many of these reactions are reversible, and on cooling,re-polymerization can occur with deposits in different forms and indifferent locations from their origins. This deposition is a commonfeature of various conventional high temperature cellulosic biomassseparation techniques. Furthermore, at higher temperatures in acidenvironments, mobilization of lignin is in competition with polymerdegradation through hydrolysis and decomposition impacting alllignocellulosic components. As a result, much effort has been expendedto devise “optimum” conditions of time and temperature that maximize theyield of particular desired products. These efforts have met with onlylimited success.

[0008] Known techniques for the conversion of biomass directly to sugaror other chemicals include concentrated acid hydrolysis, weak acidhydrolysis and pyrolysis processes. These processes are not known tohave been demonstrated as feasible at commercial scale under currenteconomic conditions or produce cellulose as either a final orintermediate product.

[0009] Conventional processes for separation of cellulose from otherbiomass components include processes used in papermaking such as thealkaline kraft process most commonly used in the United States and thesulphite pulping process most commonly used in central Europe. There areadditional processes to remove the last traces of lignin from thecellulose pulp. This is referred to as “bleaching” and a commontreatment uses a mixture of hot lye and hydrogen peroxide. Thesetechnologies are well established and economic for paper makingpurposes, but have come under criticism recently because ofenvironmental concerns over noxious and toxic wastes. These technologiesare also believed to be too expensive for use in production of cellulosefor use as chemical raw material for low value products.

[0010] The use of organic solvents in cellulose production has recentlybeen commercialized. These processes also are expensive and intended forproduction of paper pulp.

[0011] Many treatments have been investigated which involve preparatingcrude cellulose at elevated temperature for enzymatic hydrolysis tosugar. Investigators have distinguished particular process variations bysuch names as “steam explosion”, “steam cooking”, “pressure cooking inwater”, “weak acid hydrolysis”, “liquid hot water pretreatment”, and“hydrothermal treatment”. The common feature of these processes is wetcooking at elevated temperature and pressure in order to render thecellulosic component of the biomass more accessible to enzymatic attack.In recent research, the importance of lignin and hemicellulose toaccessibility has been recognized.

[0012] Steam cooking procedures typically involve the use of pressure ofsaturated steam in a reactor vessel in a well-defined relationship withtemperature. Because an inverse relationship generally exists betweencooking time and temperature, when a pressure range is stated inconjunction with a range of cooking times, the shorter times areassociated with the higher pressures (and temperatures), and the longertimes with the lower pressures. As an aid in interpreting and presentingdata from steam cooking, a “severity index” has been widely adopted andis defined as the product of treatment time and an exponential functionof temperature that doubles for every 10° C. rise in temperature. Thisfunction has a value of 1 at 100° C.

[0013] It is known that steam cooking changes the properties oflignocellulosic materials. Work on steam cooking of hardwoods by Masonis described in U.S. Pat. Nos. 1,824,221; 2,645,633; 2,294,545;2,379,899; 2,379,890; and 2,759,856. These patents disclose an initialslow cooking at low temperatures to glassify the lignin, followed by avery rapid pressure rise and quick release. Pressurized material isblown from a reactor through a die (hence “steam explosion”), causingdefibration of the wood. This results in the “fluffy”, fibrous materialcommonly used in the manufacture of Masonite™ boards and Cellotex™insulation.

[0014] More recent research in steam cooking under various conditionshas centered on breaking down the fiber structure so as to increase thecellulose accessibility. One such pretreatment involves an acidified“steam explosion” followed by chemical washing. This treatment may becharacterized as a variant of the weak acid hydrolysis process in whichpartial hydrolysis occurs during pretreatment and the hydrolysis iscompleted enzymatically downstream. One criticism of this technique isthat the separation of cellulose from lignin is incomplete. This makesthe process only partially effective in improving the accessibility ofthe cellulose to enzymatic attack. Incomplete separation of cellulosefrom lignin is believed to characterize all steam cooking processesdisclosed in prior art.

[0015] Advanced work with steam cooking in the United States has beencarried out at the National Renewable Energy Laboratory in Golden,Colorado. U.S. Pat. Nos. 5,125,977; 5,424,417; 5,503,996; 5,705,369; and6,022,419 to Torget, et al. incorporated herein by reference, involvethe minimization of acid required in the production of sugar fromcellulose by acid hydrolysis in processes that may also include the useof cellulase enzymes. These patents teach the use of an acid wash ofsolids in the reaction chamber at the elevated temperature and pressureconditions where hemicellulose and lignin are better decomposed andmobilized. The use of acid is tied to the goal of sugar production byhydrolysis. The focus of Torget's work appears to be acid treatments andhydrolysis and does not claim to produce high purity cellulose that is aprincipal objective of the present invention.

[0016] A common feature of acid hydrolysis, acid pretreatment, andchemical paper pulping is the generation of large quantities of wastechemicals that require environmentally acceptable disposal. One proposedmeans of waste disposal is as a marketable byproduct. Thus wallboard hasbeen suggested as a potential use for the large quantities of gypsumproduced in acid hydrolysis and acid pretreatment. This potential marketis believed illusory since the market for cheap sugar is so vast thatany significant byproduct will quickly saturate its more limited market.

[0017] There remains a pressing need for a process to provide low costcellulose for subsequent conversion to glucose sugar by enzymatichydrolysis. However, the presence of lignin in cellulosic biomassincreases dramatically the amount of enzyme needed, thereby imposingunacceptably high conversion costs. Economics demand a process by whichsubstantially pure cellulose can be produced for only a few cents perpound. Mainstream scientific and engineering efforts to utilizelignocellulosic biomass have been unable to achieve this goal overseveral decades. The challenge is to find a process that solves oravoids the problems of cost, chemical wastes, the clean separation oflignocellulosic components, and the unwanted degradation of saidcomponents.

[0018] Ignored by the mainstream effort is a process referred to as “wetoxidation”. This is a mature technology used for the disposal of liquidand toxic organic wastes. The process involves exposing a slurry oforganic material to oxygen at elevated temperature and pressure evenhigher than that used in steam cooking. The result is destruction of theorganic material and its conversion to carbon dioxide and water. Whilethe effectiveness of wet oxidation in the chemical modification oforganic matter has been demonstrated at commercial scale, the severityof chemical breakdown in waste disposal applications leaves few usefulproducts.

[0019] The use of wet oxidation in the pretreatment of lignocellulosicbiomass is known. In one described process, wet oxidation occurs atrelatively low temperatures (40° C.) and extends over 2 days. In otheruses of wet oxidation in the pretreatment of lignocellulosic biomass,there is no control of pH, so acids formed in the process essentiallycreate a variant of the mild acid pretreatment process.

[0020] In other wet oxidation work with wheat straw to recoverhydrolyzed hemicellulose, process temperatures were maintained from 150°to 200° C. and pH was maintained at above 5 with sodium carbonate. LowerpHs were avoided to minimize decomposition and the formation ofchemicals toxic in downstream processes. The separation of cellulosefrom lignin was not a stated goal in this work, and it is believed thatthe chemical conditions were not appropriate for such a separation.Perhaps the greatest deficiency of this work is that the entire biomasswas subjected to the same treatment for the entire processing time. Thusa compromise was needed with consideration given to both the mostreactive and the least reactive components. The resulting “optimized”procedure fails to satisfy the requirements for commercializationbecause of component degradation and low yields.

[0021] Thus it can be seen that neither technologies for paper making,for acidified steam cooking, nor for wet oxidation as presentlypracticed can fill the need for commercially economical techniques forpreparation of high purity cellulose from cellulosic biomass which donot produce objectionable waste streams.

3. OBJECTS OF THE PRESENT INVENTION

[0022] Accordingly, one object of the present invention is to provide alower cost and environmentally benign process for the separation ofcellulose from other constituents of cellulosic biomass.

[0023] Another object of this invention is to produce at high yield andin a chemically active state cellulose that is substantially free oflignin, hemicellulose, and extractives that are other constituents ofbiomass.

SUMMARY OF THE INVENTION

[0024] According to the present invention, it has been found thatrelatively pure cellulose can be produced if lignocellulosic materialsfirst are treated with steam to partially hydrolyze the hemicellulose tosoluble oligomers and then are washed with alkaline hot water containingdissolved oxygen to remove these hydrolysis products and to decompose,mobilize and remove lignin, extractives, and residual hemicellulose.

[0025] A preferred method of the present invention involves theproduction of purified cellulose containing less than 20% lignin bychemical alteration and washing of lignocellulosic biomass materialunder elevated pressure and temperature. The method includes the stepsof providing a lignocellulosic feedstock having an average constituentthickness of at most 1″, (most preferably up to ⅛″ thick), introducingthe feedstock into a pressure vessel having at least two reaction zones,heating the feedstock in a first reaction zone to a temperature of fromabout 180° C. to about 240° C., transferring said heated feedstock fromsaid first reaction zone to said second reaction zone while subjectingsaid feedstock to an oxidizing counterflow of hot wash water of pH fromabout 8 pH to about 13 pH to create a residual solid containingcellulose and a filtered wash water containing dissolved materials.

[0026] Optimum operating conditions depend somewhat on the type ofbiomass being treated, with process times being about 1 to 10 minutesand the weight of wash water used being about 2 to 20 times the dryweight of feedstock. In addition to an oxidizer, chemicals must beintroduced as necessary to maintain a pH between about 8 and 13 invarious reaction zones.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a schematic illustrating a continuous systemincorporating the techniques of the present invention in the productionof cellulose from lignocellulosic biomass.

[0028]FIG. 2 is a section view illustrating details of the operation ofthe hydrothermal wash chamber of FIG. 1.

[0029]FIG. 3 is a section view illustrating portions of a heatrecuperation subsystem of FIG. 1.

[0030]FIG. 4 is a diagrammatic view illustrating a system for practicingthe process of the invention on a semi-continuous batch basis.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0031] The present invention relates to a technique in which relativelypure cellulose is produced from lignocellulosic materials which aretreated with steam to partially hydrolyze the hemicellulose to solubleoligomers and then washed with a counter current flow of alkaline hotwater containing dissolved oxygen to remove these hydrolysis productsand to decompose, mobilize and remove lignin, extractives, and residualhemicellulose. In a preferred embodiment, a method of the presentinvention involves the production of purified cellulose containing lessthan 20% lignin by chemical alteration and washing of lignocellulosicbiomass material under elevated pressure and temperature. The methodincludes the steps of providing a lignocellulosic feedstock having anaverage constituent thickness of at most 1″, (most preferably up to ⅛″thick), introducing said feedstock into a pressure vessel having atleast two reaction zones, heating said feedstock in said first reactionzone to a temperature of from about 180° C. to about 240° C.,transferring said heated feedstock from said first reaction zone to saidsecond reaction zone while subjecting said feedstock to a counterflow ofhot, alkaline wash water from about 8 pH to about 13 pH to create aresidual solid containing cellulose and a filtered wash water containingdissolved materials.

[0032] More particularly, the preferred feedstock is sawmill waste inthe Pacific Northwest consisting of sawdust, bark, chips, hog material,and the like. It should be understood, however, that the techniques ofthe present invention are effective on a broad range of lignocellulosicmaterials including but not limited to wood, grass, herbacious crops,agricultural waste, waste paper, high cellulosic industrial solid wasteand municipal solid waste.

[0033] The operation of a system in which the present invention may bepracticed is shown in FIG. 1. Lignocellulosic biomass feedstock (1) issubjected to a preliminary preparation (2) as required by the particularnature of the feedstock. In the case of moist sawdust, for example, nopreparation of any kind might be needed. In the case of municipal solidwaste, a materials recycling facility may be the source of a feedstockstream substantially free of extraneous materials. The preferred averageconstituent thickness of the feedstock material is at most 1″ inthickness. Constituent size is controlled by mechanical treatment of thefeedstock material by chipping, grinding, milling, shredding or othermeans. The most preferred average constituent size is not greater thanabout ⅛″ thick by 1″ in other dimensions. For sawmill waste, aconventional chipper provides adequate size reduction.

[0034] The prepared feedstock is next preheated (3) by steam and forcedmechanically (4) into the hydrothermal wash chamber (5) where it isfurther heated by steam injection (6) to a temperature of about 220° C.and a pressure of about 340 psia. As solids pass through the washchamber, lignin and hemicellulose are mobilized and separated from thecellulose. The cellulose is discharged from the wash chamber at (7) intothe flash tank (8) and delivered as product (9). Steam generated in theflash cooling (10) is recycled to preheat the solids feed.

[0035] Wash water from a reservoir (11) is pumped (12) into the heatrecovery system (14) where it is preheated before entering the heater(16) where alkali (22) and steam (17) are injected to raise thetemperature to about 220° C. for injection into the hydrothermal washchamber (5) at (18). This hot, pressurized wash water flows counter tothe movement of solids, collecting lignin and hemicellulose and leavingthe wash chamber at (19) where it passes through the heat recoverysystem to provide preheat for the wash water. The weight of wash waterused typically will be about 5 times the dry weight of feedstock.

[0036] The process of this invention requires that the wash water bealkaline. This is accomplished by adding lime at (22) in sufficientquantity to provide near saturation. This will require about 0.3 kgm oflime per metric ton of wash water. In addition, lime will be consumed inneutralizing acids that may be formed by oxidation. An additional kgm ormore of lime per dry metric ton of feed may need to be introduced as aslurry at (23). The auger action mixes the lime with the feed to beconsumed as needed in the process. The precise amount to be added willdepend, in part, on the nature of the feedstock. The cellulose productmust be monitored to insure that little or no lime carries through theprocess, and the wash liquor must be monitored to insure that a pH of atleast 11 is maintained. The flow of lime slurry is then adjusted to meetthese two requirements.

[0037]FIG. 2 illustrates operation of the hydrothermal wash chamber inmore detail. This apparatus consists of a cylindrical pressure vessel(24) containing a rotal auger (25) driven by a motor (26). Provision ismade for forced insertion of solid material (4) at (27) and forintermittent discharge of solids (7) through a ball valve (28). Theinsertion and release of solids is accomplished without significant lossof chamber pressure. This entire apparatus, known as STAKE II, iscommercially available from Stake Technology, Ltd., a Canadiancorporation, and can be sized for any application. Functionallyequivalent apparatus is available elsewhere and is in common use in thepaper industry. Some such apparatus may employ twin screws eitherco-rotating or counter-rotating. For the present application, thestandard STAKE II design is modified by the manufacturer to include ascrew having interruptions and/or different pitch on the two ends (toaccommodate the dissolving of a portion of the feed) and having portsfor the injection and discharge of pressurized liquid. Auger actioncompacts solids at the discharge end to minimize loss of wash waterduring solids release. The auger action also subjects solids to shearingforces. As material dissolves, the remaining solid is weakened, and theshear forces break up the larger pieces, expose more surface area, andso facilitate further dissolution.

[0038] In the preferred implementation, pressurized wash water isinjected at (17) and exits at (29) to a gas trap (30) where air trappedin the feed and gasses introduced or released in the processing can bevented (31). The wash liquor containing dissolved lignin andhemicellulose then continues its flow under pressure at (19). It isnecessary that the drain (29) be equipped with a filter in the wall ofthe wash chamber to prevent solids from escaping. The fresh solids aredriven by the auger at close spacing to scour the filter and prevent thebuildup of fines that could cause clogging.

[0039] An important feature of the process of the present invention isthe control of pH. Either acid or base can catalyze hydrolysis and otherirreversible chemical reactions. As steam (6) heats the fresh solids,acetic acid is released from degradation of the hemicellulose and canreduce the pH to as low as 3. In the preferred implementation, thisacidity auto-catalyzes the hydrolysis of some hemicellulose to solubleoligomers. The goal is to hydrolyze the hemicellulose and then quicklyto raise the pH and wash the resulting oligomers out of the chamber at(29) in order to prevent further degradation. The hemicellulose spendsnot more than about 30 to 60 seconds in the wash chamber, and duringthis brief time, the steam has little effect on the lignin andcellulose. As a variation on this procedure, steam injected at (6) mightraise the temperature to as little as 180° C. for a more extended timeto hydrolyze hemicellulose after which additional steam injected at (35)could further increase the temperature to dissolve lignin.

[0040] In the wash zone between (17) and (29), the goals are first tomaintain alkaline conditions in order to prevent hydrolysis ofcellulose, prevent condensation of lignin, and promote dissolution ofthe lignin and wash it away. To maintain a proper pH, lime or other baseis injected with the wash water (18) and in a slurry at (29). At (29)the lime is injected both before and after the liquid discharge at (29)in order to avoid waste. The flow just before (29) is adjusted to theminimum required to neutralize acids formed in zone one and to raise thepH to about 11 or 12. The flow (23) just after (29) should be sufficientto neutralize all acids formed in the following zone(s).

[0041] In other implementations, more complex wash patterns may beemployed such as feeding wash water at (17) with an exit at (36) toremove lignin while providing a second feed of wash water at (35) withexit at (29) to remove hemicellulose. Interruption of the screw between(36) and (29) and perhaps modifying the cross section of the washchamber wall could then provide a moving barrier of compacted solids tominimize mixing of liquids. The common innovation in all implementationsof the present invention is the washing of cellulose solids at hightemperature and pressure under alkaline conditions that minimizeundesirable chemical degradation.

[0042] Oxygen is injected in controlled amounts and at controlledpressure at positions (32) and (34) and at intervening position (notshown) as required, depending on apparatus size and other factors. Inthe preferred implementation, at least 3 kgm of oxygen may be requiredfor each dry metric ton of feed. The total reaction time depends on thespeed of the auger drive motor (26) and will be between 2 and 4 minutesin the preferred implementation. Motor speed, temperature, water wash,and oxygen flow rate can be adjusted to optimize cellulose production.

[0043] Heating large volumes of wash water to high temperatures isenergy intensive. FIG. 3 illustrates an energy conservation feature.Wash liquor carrying dissolved solids from the wash chamber isdischarged (19) to a chain of flash tanks (57), (53), (45) for stepwisereduction of pressure to atmospheric. Each flash tank is paired with acondensing heat exchanger (55), (49), (42) that is part of a chain topreheat the wash water to the wash chamber. Flash cooling of liquid(19), (54), (48) entering each flash tank generates steam (56), (52),(44) that flows to the heat exchangers where it condenses. Thiscondensed liquid (51), (47) is then flashed to the next heat exchangerin the chain. Thus the total wash liquor plus flash liquor being flashcooled at each stage remains constant. Flash tanks are of a standarddesign, and heat exchangers are of standard design—all apparatus sizedfor the particular application and rated for the required pressure.

[0044] The heat of the final condensed flash liquor (41) at about 100°C. is used entirely or in part to preheat wash water in theliquid-liquid heat exchanger (39). This flash condensate may containsome volatile chemicals but is not particularly corrosive.

[0045] To insure proper operation, pressure in the flash tanks iscontrolled with a control system in which the measured pressure and/ortemperatures in the various flash tanks and heat exchangers are used toregulate variable nozzles that admit liquid continuously to the tanks.

[0046] Wash water (13) from the feed pump (12) flows through heatexchangers (39) and (42) that operate near atmospheric pressure attemperatures below 100 degrees C. Pressure pump (46) then increasespressure to that required for the hydrothermal wash—about 450 psia inthe preferred embodiment. The wash water continues its flow through heatexchangers (49) and (55) to the final heater (16) where it is brought tofinal temperature by steam injection (17). When three stages of flashcooling are used as shown, the wash water heating requirement is reducedby over 75%. If an additional stage of flash cooling with heat exchangeris added between (53) and (57) the wash water heating requirement isreduced by over 80%. The choice of the number of flash cooling stages tobe used in any application involves the balancing of capital andoperating costs.

[0047] More particularly, optimal energy recycle depends on a number offactors that translate ultimately to a sequence of operating pressuresfor the flash tanks. Factors that must be considered include:composition of the solid feed material to the cellulose recovery processincluding moisture content, temperature of this feed, processingtemperature, dilution of dissolved solids, moisture content ofdischarged cellulose, and temperature drop across the heat exchangers.In the course of computation it is generally found that the flow ofeffluent wash water is not the same as the input of fresh wash watersince moisture from the solids feed and from steam condensate has beenadded and moisture in the cellulose output has been subtracted. Inaddition, there are the dissolved solids to consider. Because of allthese dependencies, an automated system is needed for minute by minuteheat exchanger control, but set points need first to be calculated insetting up the system.

[0048] Calculation begins with mass balances for the hydrothermal washchamber. With reference to FIG. 3, let the flow of fresh wash water intothe reaction chamber at (18) be Wr and the flow of wash liquor withdissolved solids out of the reaction chamber at (19) be Lr. The ratio ofthese two flows is an important determinant of the flash tank pressures,so define R=Wr/Lr. Values for Wr, Lr, and R must be calculated from theoperational requirements of the process application. Thus start with thefeed rate of solid material, its temperature, and its composition,consider the steam flow required to heat to operating temperature,consider the portion of the feed that will be dissolved, consider themoisture content of the solids to be discharged, and consider theallowable concentration of dissolved solids in the effluent wash liquor.With knowledge of the heat capacities of the various materials and byuse of a set of steam tables the necessary calculations can beperformed. For fresh sawmill waste in the Pacific Northwest the resultwill be R=0.8 more or less.

[0049] For initial process design purposes, an approximate calculationcan be done to determine stage temperatures and mass flows. Consider theliquid flow, Lr, at temperature, Tr, with enthalpy Hr at (19) in FIG. 3.This liquid is to be cooled to temperature, T3, and enthalpy H3 in flashtank FT3 (57). The enthalpy change will be Lr*(Hr−H3). This excess heatwill flash part of the liquid to steam with an enthalpy change fromliquid to vapor given by F3*HIv3=Lr*(Hr−H3) where F3 is the rate ofsteam flow at (56). This flash steam will pass to the heat exchanger HE3(55) where it will be condensed and give up its heat to the fresh washwater Wo (13): Wo*(H3−H2)=F3*HIv3. Similar relationships can be writtenfor all stages. For ease of calculation, liquid enthalpies in Btu/poundcan be expressed approximately as H=1.8*T degrees C. It is alsoconvenient to normalize mass flows with respect to the fresh wash waterfeed, Wo, so that L=Lr/Wo, S=Sr/Wo, W=Wr/Wo, fs=Fs/Wo where “s” is stagenumber. Thus a set of equations can be written to describe the heatexchange process: Steam Input: W = 1 + S (50) S*HIvr 18*(Tr − T3 + D)(51)

[0050] where “D” is the temperature drop across any heat exchanger.Stage 1: T2 − T1 = (T1 − To)/L (52) f1*HIv1 = L*1.8*(T2 − T1) (53) Stage2: T3 − T2 = (T2 − T1)/L (54) f2*HIv2 = L*1.8*(T3 − T2) (55) Stage 3: Tr− T3 = (T3 − T2)/L (56) f3*HIv3 = L*1.8*(Tr − T3) (57)

[0051] The pattern can be extended to any number of stages, from which:

S=W−1=L*R−1  (58)

D=(HIvr*S)/1.8−(Tr−T1)*(L−1)/(L ^(n)−1)  (59)

T1−To=(Tr−T1)*(L−1)/(1−L ^(−n))  (60)

[0052] Where: n=3 is the number of stages in the preferredimplementation.

[0053] Computation is facilitated if a certain sequence is followed:First, set the reactor temperature, Tr, and the temperature of the firststage, T1=100. With Tr, T1, and R specified pick a trial value for L>1from which get S (58) and D (59). Adjust L until the value for D isacceptable—usually about 10. Then calculate (T1−To) (60), (T2−T1) (52),(T3−T2) (54), etc. through all stages. Obtain values for theliquid-to-vapor enthalpy changes, HIv, from steam tables and calculatef1, f2, f3, etc. through all stages and sum for the total flash liquorfo. From the mass balance on the wash chamber, Lr will be known, so useWo=Lr/L to remove the normalization from mass flows for steam andliquids. With T1 known, calculate To, T2, T3, etc. and determinecorresponding flash tank pressures (Ps in pounds per square inch) fromthe steam tables. For example, with Tr=220 C: T3=192, P3=188, T2=153,P2=74, more or less.

[0054] Although optimization of the operating parameters in the practiceof the present invention provides additional economic and otherbenefits, the techniques of the present invention provide morefundamental benefits which can be readily appreciated. Because littleuse is made of chemical additives in the processes of the presentinvention, waste disposal problems are minimized. Furthermore, theeffluent wash water liquor includes lignin, oligomers and monomers fromhemicellulose and extractives that are relatively free of toxicdegradation products and may be further processed for their economicvalues. In addition, energy recuperation is achieved through use of heattransfer between output and input streams to minimize the cost ofheating wash water.

[0055] Batch type experiments utilizing corn stover (stems, leaves,cobs, shucks, etc.) were conducted in the laboratory to simulate acontinuous process for commercial production of purified cellulase fromcommon waste biomass. In the continuous process, a single elongatedreaction vessel may be used in which liquid and solids move in oppositedirections in some zones and in the same direction in others with thesolids passing through as sequence of reactive conditions.

[0056] For the laboratory experiments, two reaction vessels were used.In the first reaction vessel, granular solid feedstock prepared in ahammer mill were loaded as a fixed bed adapted for soaking and/orwashing at elevated temperature and pressure using one or more liquidpreparations. Conducted in the first reaction vessel are the steps ofthe process in which hemicellulose and extractives are mobilized andeluted and in which, under changed conditions, most of the lignin iseluted.

[0057] In the second reaction vessel, residual lignin was eliminatedwith a “polishing” step. This second reaction vessel used in thelaboratory experiments was a simple “bomb” into which solids from thefirst reaction vessel were loaded along with water, alkali, andpressurized oxygen. The “bomb” was then heated to initiate oxygenationand quenched to end the run.

EXAMPLE I PVT-12

[0058] 13.2 grams (dry weight) feedstock of feedstock were placed in thefirst reaction vessel. Water was added, the slurry heated and the heatedslurry maintained at a peak temperature of 170° C. for 5 minutes. Theslurry was then washed with 900 grams of heated water previously treatedwith NaOH to pH 12.8. Maximum temperature of the wash water was measuredat 224° C. Log severity=4.5 Product results were measured withouttransfer to and treatment in the second reaction vessel. Recoveryresults are summarized as follows:

[0059] Solid product: Recovery=44.2% by wt. of feed material

[0060] Recovery of original cellulose with composition>98%

[0061] 92.4% 6-carbon components (mostly cellulose)

[0062] 0.3% 5-carbon components

[0063] 3.3% Klason lignin

[0064] 0.2% acid soluble lignin

[0065] 3.8% ash.

EXAMPLE II PVT-Combo

[0066] First Vessel:

[0067] 53.1 grams (dry weight) feedstock distributed in 4 runs in thefirst vessel. As much as possible, conditions of the PVT-12 run wereduplicated for each run, and the products were combined to providematerial for studies of oxidation in the second vessel.

[0068] Combined product recovery=37.9% by wt of feed material

[0069] 1.9% Klason lignin

[0070] 3.2% ash.

[0071] Second Vessel:

[0072] Two samples of combo product of about 1 gram each were treatedwith oxygen as described above at a peak temperature of 217° C. and alog severity of 4. Oxygen pressure in one sample was 50 psig and in theother was 60 psig. Results from these two runs were indistinguishable,so only averages follow.

[0073] Solid product: Recovery=31.9% by wt. of feed material

[0074] Recovery of original cellulose with composition≈82%

[0075] 96.4% 6-carbon components (mostly cellulose)

[0076] 5-carbon components not detectable

[0077] 0.7% Klason lignin

[0078] acid soluble lignin not detectable

[0079] 2.9% ash.

EXAMPLE II PVT-19

[0080] First Vessel:

[0081] 12.0 grams (dry weight) feedstock. Initial Water Soak: 5 minutesat peak temperature of 200° C. Washed with 370 grams of distilled waterat 200° C. followed by 865 grams of water at pH 12.8 with NaOH. Maximumtemperature=218 C. Log severity=4.1.

[0082] Second Vessel:

[0083] All solid material from first vessel transferred to second vesseland soaked in a pH 12.8 solution with 150 cc of oxygen at STP and oxygenpressure of 45 psig. Maximum temperature=215° C. Log severity=3.8.

[0084] Solid product: Recovery=31.4% by wt. of feed material

[0085] Recovery of original cellulose with composition≈82%

[0086] 97.5% 6-carbon components (mostly cellulose)

[0087] 5-carbon components not detectable

[0088] Klason lignin not detectable

[0089] acid soluble lignin not detectable

[0090] 2.5% ash.

[0091] As can be seen, using the process of the present invention, therecovered cellulose consistently contains less then 20% lignin, aspredicted, with the two reaction zone technique of the present inventionconsistently yielding recovered cellulose containing less than 10% to 5%lignin. Indeed, the two reaction zone technique of the present inventionis shown above to yield recovered cellulose containing less than 2%lignin and less than 1%, which is most preferred.

[0092] While the processes herein described and the forms of apparatusfor carrying these processes into effect constitute preferredembodiments of this invention, it is to be understood that the inventionis not limited to these precise processes and forms of apparatus andthat changes may be made in either without departing from the scope ofthe invention which is defined in the appended claims.

I claim:
 1. A method of treating lignocellulosic biomass containinglignin to produce purified cellulose, said method comprising: providinga lignocellulosic feedstock; introducing said feedstock into a pressurevessel having at least two reaction zones; heating said feedstock in afirst reaction zone to a temperature of from about 180° C. to about 240°C.; removing hydrolized hemicellulose from the heated feedstock;transferring said heated feedstock from said first reaction zone to asecond reaction zone; subjecting said feedstock in said second zone to acounterflow of hot wash water containing dissolved oxygen and having apH of at least 11 to produce residual solids containing cellulose and awash water containing lignin and other extractives; and separating theresidual solids containing purified cellulose from the filtered washwater, wherein the separated cellulose contains less than 10% lignin. 2.The method of claim 1, wherein the separated cellulose contains lessthan 5% lignin.
 3. The method of claim 1, wherein the separatedcellulose contains less than 2% lignin.
 4. The method of claim 1,wherein the separated cellulose contains less than 1% lignin.